Calcium-siRNA nanocomplexes: what reversibility is all about

Gene silencing using small interfering RNA (siRNA) relies on the critical need for a safe and effective carrier, capable of strong but reversible complexation, siRNA protection, cellular uptake, and cytoplasmatic unloading of its cargo. We hypothesized that a delivery platform based on the eletrostatic interactions of siRNA with calcium ions in solution would fulfill these needs, ultimately leading to effective gene silencing. Physical characterization of the calcium-siRNA complexes, using high resolution microscopy and dynamic light scattering (DLS), showed the formation of stable nanosized complexes ~80nm in diameter, bearing mild (~-7mV) negative surface charge. The complexes were extremely stable in the presence of serum proteins or high concentrations of heparin; they maintained their nanosized features in suspension for days; and effectively protected the siRNA from enzymatic degradation. The Ca-siRNA complexes were disintegrated in the presence of Ca-chelating ion exchange resin, thus proving their reversibility. Excellent cytocompatibility of calcium-siRNA complexes was achieved using physiological calcium ion concentrations. The calcium-siRNA complexes successfully induced a very high (~80%) level of gene silencing in several cell types, at both mRNA and protein levels, associated with efficient cellular uptake. Collectively, our results show that the developed delivery platform based on reversible calcium-siRNA interactions offers a simple and versatile method for enhancing the therapeutic efficiency of siRNA.